Several fluid, genetic, and neuroimaging biomarkers have been associated with the development of cognitive decline in PD, such as cerebrospinal fluid (CSF) amyloid β, apolipoprotein E4, single-nucleotide polymorphisms (SNPs) such as BDNF Val/Val and COMT Val/Val, reduced global brain volume, a brain atrophy pattern resembling AD, and reduced uptake of dopamine transporter (DAT) in the caudate nucleus [8,9]. This evidence concerns the gene SLC6A3 and Alzheimer disease.